The present invention relates to a method for controlling the path of movement of a workpiece socket head of an orbital motion welding system as well as to a welding system for performing the method.
Friction welding systems weld workpieces in particular of plastic materials by applying pressure to the workpieces and causing them rapidly to move so as to melt the contact areas of the workpieces without the necessity of adding welding materials. The frequency of such mechanical movement lies in the magnitude of some 100 Hertz. Commonly a system having a movable ferromagnetic mass supported on springs and actuated by several electromagnets is used to generate such movements. Such system is generally driven at a frequency near its mechanical resonance frequency.
Friction welders which perform reciprocating movements along only one axis (so-called linear friction welding systems) are well known in the art. DE 25 39 167 C3 and U.S. Pat. No. 3,920,597 describe such linear welding systems. In such systems it is only necessary to control the amplitude of the displacement along the path of movement. This is accomplished mechanically, and the timing of the movement sometimes referred to as track curve is defined by the mechanical resonance. However, the welding zones show visual bulging of material in the direction of the movements. Furthermore, the movements come to a stand still at the turning points so as to reduce the average velocity.
Therefore, friction welding systems using orbital movements, in first approximation circular movements were developed. EP 0 504 494 A2 and U.S. Pat. No. 5,160,393 which are incorporated herein by reference disclose such systems. Such systems provide for less bulging at the welding area for the bulged material is spread more uniformly at the periphery of the contact area. Furthermore it is possible to obtain the same mean velocity and the same heating power at an amplitude which is 29%(.sqroot.2/2) less than with linear welders. The electromagnets in such welding systems are usually driven by commercially available frequency converters which also control the amplitude of the movements as it is known from DE 25 39 167.
However, the path of movement is determined essentially by the mechanical system. Therefore, a circular path is obtained only if not only the tools holding the workpieces but also the workpieces themselves are almost rotationally symmetric with respect to the axis of rotation. Generally, these conditions are not fulfilled. Therefore, workpieces that are not rotationally symmetric will result in an elliptical path of movement will result so that the advantages of orbital movements cannot or only partly be obtained with such workpieces. Therefore, it would be advantageous to have an active control of the path of movement.